1. Field of the Invention
The present invention relates to lead-free piezoelectric ceramic materials comprising crystalline (and preferably perovskite crystalline) structures of the formula Bi1-x(RE)xFeO3, where RE is one or more of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and 0≦x≦0.3. The materials are at or near the morphotropic phase boundary and display enhanced piezoelectric and dielectric properties.
2. Description of Related Art
A. The Piezoelectric Effect and Piezoelectric Materials
Piezoelectricity relates to the ability of certain non-conductive crystalline materials to develop an electrical charge in response to and proportional to mechanical stress, and mechanically deform upon application of electric field Piezoelectric materials are discussed in U.S. Pat. Nos. 4,400,642; 4,560,737; 5,230,921; 5,621,264; 6,447,887; 6,515,404; 6,987,433; and 7,479,728.
A piezoelectric material consists of multiple interlocking domains which have positive and negative charges. These domains are symmetrical within the material, causing the material as a whole to be electrically neutral. When stress is put on the material, the symmetry is slightly broken, generating voltage. Even though a piezoelectric material never deforms by more than a few nanometers when a voltage is applied to it, the force behind this deformation is extremely high, on the order of mega-newtons. The property of piezoelectricity reflects both the atomic constituents of the material and the particular way in which the material was formed.
The piezoelectric effect is reversible in that materials exhibiting a direct piezoelectric effect (i.e., the production of electricity when stress is applied) also exhibit the reverse piezoelectric effect (i.e., the production of stress and/or strain when an electric field is applied).
Piezoelectric materials can be divided in 2 main groups: crystals and ceramics. Piezoelectric ceramics are composed of small grains (crystallites), each containing domains having aligned electric dipoles. Piezoceramic materials have several advantages over single crystals, including; higher sensitivity and the ability to be more easily fabricated into a desired shape and size. Piezoelectric ceramics are used in a broad range of applications due to their excellent properties of high sensitivity, ease of manufacture and the possibility of poling the ceramic in any direction. Applications of piezoceramics include accelerometers, acoustic emission transducers, actuators, alarm systems, speakers, movement detectors, broken window sensors, dental work: removal of plaque, flow meters: blood, industrial process, waste water; hydrophones: seismic, biologic, military, underwater communication; industrial sensors based on ultrasound: level control, detection, identification; inkjet printers; medical: scanning, heat treatment, surgical knives, cleaning blood veins; micro positioning devices: optics, scanning tunneling microscopes; musical instruments pickups; surface acoustic waves: personal computer touch screens, filters; underwater acoustics; and welding and drilling of metals and plastics. Piezoelectric ceramics are discussed in U.S. Pat. Nos. 5,637,542; 5,914,068; 6,004,474; 6,231,779; 6,358,433; 7,090,785; and 7,468,143.
B. Characteristics of Piezoelectric Materials
Normal ceramics are not piezoelectric because the random orientation of their individual crystallites imparts an infinite degree of rotational symmetry within the ceramic texture. In contrast, piezoelectric materials exhibit a substructure of electrically polar crystallite domains that can be reoriented by a strong applied electric field. This property is know as “ferroelectricity” (Batthais, B. T. et al. (1948) “Domain Structure and Dielectric Response of Barium Titanate Single Crystals,” Phys. Rev. 73:1378-1384; von. Hippel, A. (1950) “Ferroelectricity, Domain Structure, and Phase Transitions of Barium Titanate,” Rev. Modern Phys., 22:221-237).
This domain reorientation is demonstrated by the appearance of electric hysteresis and significant shape change in the presence of an electric field. When an electric field is applied to a ferroelectric material, the material expands if the field is parallel to the axis of the material's polarization, and contracts if the field is anti-parallel to this axis. This response is known as the “piezoresponse” of the material.
A ferroelectric material may also undergo a transition to an antiferroelectric state in a piezoceramic material. In an antiferroelectric transition, individual dipoles become arranged anti-parallel to adjacent dipoles with the result that the net spontaneous polarization is zero. Thus materials in their antiferroelectric states generally have low dielectric constants of about 100 to about 1000. This antiferroelectric phase may exist at room temperature, and is generally associated with a structural phase transition from the antiferroelectric state to a ferroelectric phase upon application of an electric field. Similarly, a ferroelectric-to-antiferroelectric phase transition may be accomplished by applying an activating electric field. Thus, piezoelectric materials are “field-tunable,” such that they can undergo a phase transition from a low dielectric state (antiferroelectric state) to a high dielectric state (ferroelectric state) upon being exposed to a biasing electric field. These advantageous properties of the antiferroelectric particles permit the composition to be field tunable. Field tunable compositions can advantageously have their dielectric properties adjusted upon demand, depending upon the application for which they are to be used.
Ferroelectricity and antiferroelectricity can exist in a number of crystal structures and compositions within those structures. Among the most important of such structures are those capable of forming the crystal lattice structure of perovskite (CaTiO3), a non-ferroelectric material. “Perovskite-type” ceramic crystals have the general formula ABO3, where A and B each represent cations and O represents oxygen. A and B differ in that A has a larger ionic radius than B and is in twelve-fold coordination, whereas B is in an octahedral six-fold coordination. Most of the useful piezoelectric and ferroelectric ceramics, such as barium titanate, (BaTiO3) potassium niobate (KNbO3), and lead titanate (PbTiO3) have perovskite-type structures. An example of a specific perovskite family comprises the structure RT3M, where R is a rare earth or other large ion, T is a transition metal ion, and M is a light metalloid. Perovskite materials exhibit colossal magnetoresistance, ferroelectricity, superconductivity, charge ordering, spin-dependent transport and high thermopower. They are thus exemplary candidates for memory devices and spintronics applications.
The piezoelectric effect for a given item depends on the type of piezoelectric material and the orientation of its mechanical and electrical axes of operation. In piezoceramics, these axes are set during the process, known as “poling, in which the ceramic's piezoelectric properties are induced. Under conditions that confer tetragonal or rhombohedral symmetry, each crystal has a dipole moment. At a particular temperature, known as the “critical temperature” (“Tc” or “Curie point”) each perovskite crystal in the ceramic exhibits a simple cubic symmetry with no dipole moment. This phase change is accompanied by a peak in the dielectric constant and a complete loss of all piezoelectric properties. At temperatures below the Curie point, each crystal has tetragonal or rhombohedral symmetry and a dipole moment. However, the direction of polarization among neighboring domains is random, and so the ceramic element itself has no overall polarization.
In order to impart a permanent polarization to the ceramic element, it is necessary to subject the ceramic to a direct current (“DC”) electric field. This “poling,” process exposes the domains of the ceramic to a direct current electric field at a temperature slightly below the Curie point and thus causes the domains to align with one another. The domains that are most closely aligned with the electric field expand, and the ceramic lengthens in the direction of the field. When the field is removed, most of the dipoles remain locked into a near-alignment configuration, resulting in permanent polarization (the “remanent” polarization) and permanent elongation. The orientation of the direct current poling field determines the orientation of the mechanical and electrical axes. The poling field can be applied so the ceramic exhibits piezoelectric responses in various directions or combination of directions. The poling process permanently changes the dimensions of the ceramic.
Mechanical compression or tension on the poled piezoelectric ceramic changes its dipole moment and thus creates a voltage. Conversely, if a voltage is applied to a ceramic element in the direction of the poling voltage, the element will lengthen and its diameter will become smaller. If a voltage of opposite polarity to the poling voltage is applied, the element will become shorter and broader. If an alternating voltage is applied, the element will lengthen and shorten cyclically, at the frequency of the applied voltage. This principle is utilized in applications such as piezoelectric motors and sound/ultrasound generating devices.
The most useful piezoelectric/ferroelectric perovskite-type ceramics display a transition region (known as the “morphotropic phase boundary” or “MPB”) in their composition phase diagrams. The morphotropic phase boundary separates regions of tetragonal symmetry from those of rhombohedral symmetry in compositionally varying ferroelectrics (Jaffe, B. et al. (1954) “Piezoelectric Properties of Lead Zirconate-Lead Titanate Solid-Solution Ceramics,” J. Appl. Phys. 25:809-810). At this transition region, the crystal structure changes abruptly and the electromechanical properties are maximal. It has been observed experimentally that the maximal values for dielectric permittivity, as well as the electromechanical coupling factors and piezoelectric coefficients of various piezoceramic compositions occur at the MPB.
Transitions through the MPB are sometimes mediated by intermediate phases of monoclinic symmetry (Noheda, B. et al. (1999) “A Monoclinic Ferroelectric Phase in the Pb(Zr1-xTix)O3 Solid Solution,” Appl. Phys. Lett. 74:2059-2061.), and the high electromechanical response in this region is related to this phase transition because of symmetry-allowed polarization rotation (Guo, R. et al. (2000) “Origin Of The High Piezoelectric Response In PbZr1-xTixO3,” Phys. Rev. Lett. 84:5423-5426; Noheda, B. et al. (1999) “A Monoclinic Ferroelectric Phase in the Pb(Zr1-xTix)O3 Solid Solution,” Appl. Phys. Lett. 74:2059-2061; Fu, H. et al. (2000) “Polarization Rotation Mechanism For Ultrahigh Electromechanical Response in Single-Crystal Piezoelectrics,” Nature 403:281-283; Cohen, R. E. (2006) “Relaxors Go Critical,” Nature 441:941-942).
Lead oxide based piezoceramics, especially lead zirconate titanate (Pb(Zr,Ti)O3) or “PZT” are presently the most widely used materials for piezoelectric actuators, sensors and transducers due to their excellent piezoelectric properties. Such piezoceramics exhibit some of the highest piezoelectric coefficients of any of the piezoceramic materials, and therefore, have been widely used in transducers, actuators and other electromechanical devices (Park et al. (1997) “Ultrahigh Strain And Piezoelectric Behavior In Relaxor Based Ferroelectric Single Crystals,” J. Appl. Phys. 82:1804-1811; Sabolsky, E. M. et al. (2003) “Piezoelectric Properties Of <001> Textured Pb(Mg1/3Nb2/3)O3—PbTiO3 Ceramics,” Appl. Phys. Lett. 78:2551-2553: Berlincourt, D. et al. (1963) “Release Of Electric Energy In PbNb(Zr, Ti, Sn)O3 By Temperature—And By Pressure-Enforced Phase Transitions,” Appl. Phys. Lett. 3:90-92).
However, the lead oxide content of PZT is nearly 60 to 70% of its total mass. Lead oxide vaporizes during processing. Additionally, lead persists in the environment for extended time periods. It accumulates in living organisms, causing brain and nervous system damage. Lead oxide toxicity has therefore led to a growing concern about using lead oxides, particularly in consumer electronics. Additionally, lead-based materials are unsuitable for use at high temperatures (e.g., temperatures above 600° C.). Therefore, a need exists for lead-free ferroelectric/antiferroelectric materials capable of operating at high temperatures.
Recently, however, sodium bismuth titanate-based and sodium niobate-based materials have been proposed as alternatives to PZT (see U.S. Pat. No. 6,093,338; US Patent Publication No. 20070228318). U.S. Pat. No. 6,507,476 discloses a tunable ferroelectric capacitor that contains a mixture of sodium bismuth titanate, barium titanate, barium strontium niobate and potassium niobate. U.S. Pat. No. 6,793,843 discloses materials that contain sodium bismuth titanate, barium titanate and sodium niobate. However, one of the major obstacles to the use of these compounds as an alternative to PZT is their high current leakage which allows current to pass through them when a high voltage is applied. Attempts have been made to improve the electrical properties of such ceramics by doping the ceramics with rare earth elements such as lanthanum (La), samarium (Sm), gadolinium (Gd), terbium (Tb) and dysprosium (Dy) etc. To date, reported lead-free piezoceramics are either Aurivilius layered compounds or alkaline niobates (Saito, Y. et al. (2004) “Lead-Free Piezoceramics,” Nature 432:84-87; Demartin, M. et al. (2004) “Lead Free Piezoelectric Materials,” J. Electroceramics 13:385-392; Hollenstein, E. et al. (2005) “Piezoelectric Properties Of Li- And Ta-Modified (K0.5Na0.5)NbO3,” Appl. Phys. Lett. 87:182905-1 to 182905-3) with complex crystal structures, which are difficult to synthesize.
While none of the presently available lead-free materials have been shown to match the overall performance of PZT, several classes of materials are now being considered as potentially attractive alternatives to PZT for special applications. The families of potassium sodium niobate and bismuth sodium titanate have the advantages of low density, low dielectric constants, high coupling coefficient (kt), and higher mechanical strength than lead-based ceramics, making them ideally suited for such applications as high frequency transducers. Additionally, their lower acoustical impedance and their low toxicity are advantageous. However, in order to capitalize on these advantages, it is necessary to carefully control their processing.
Thus, despite all such advances, a need exists for lead-free piezoceramic materials that display ferroelectric to antiferrolectric transitions, and display robust piezoelectric properties.